Optical information recording media are known as media with which large amounts of information can be recorded and reproduced at high speeds. Optical information recording techniques record by utilizing changes in a recording material into differing optically-distinct states caused by the heat generated when the recording material is irradiated by a laser light. Such recording media have the significant advantages of making it possible for random access when necessary while also being highly transportable, and thus are becoming more and more important in recent times. Rewriteable media, which can be rewritten multiple times, and write-once media, which can only be written once, can be given as types of optical information recording media that have been conventionally proposed.
In general, write-once media often have fewer layers than rewriteable media, making it possible to manufacture the media easily and at low cost. Furthermore, because write-once media cannot be rewritten, they are favorable when writing data that a user does not wish to be destroyed. For these reasons, write-once media, a highly-reliable media whose storage lifespan is long, are in significant demand for archival applications. Demand for write-once media is increasing in various areas, such as, for example, the recording and storage of personal data and video information through computers, use in medical and academic fields, and the replacement of household video tape recorders. Meanwhile, due to the higher performance of applications, an increase in the quality of image information, and so on, even larger capacities and higher speeds are being called for in such recording media.
There is currently a particular increase in demand for writing information at high speeds in order to increase the convenience for consumers who use optical information recording media. Accordingly, it is necessary for the laser used in the recording of the information to be more powerful than before. However, if a laser is to be used in a stable manner over a long period of time, there is a limit to the power that can be introduced to that laser.
Furthermore, when information is recorded onto an optical information recording medium at high power, a greater thermal load is applied to the dielectric layer or the like that makes contact with the recording layer, resulting in a problem that the reliability of the recording medium decreases. In other words, increased sensitivity is desirable from the standpoint of recording medium reliability as well.
Multilayer media, in which information is recorded onto multiple layers, are becoming necessary for the purpose of improving recording density. With multilayer optical information recording media, which have two or more information layers, it is necessary to record data onto the information layers farther from the light entrance surface through the information layers closer to the light entrance surface, and thus a higher recording sensitivity is required. Conversely, it is necessary to reduce the thickness of the recording layers in order to realize a higher transmissibility in the information layers closer to the light entrance surface, and thus recording layers with higher sensitivity are required. Due to such factors, higher sensitivities are being demanded in multilayer recording media that attempt to realize higher capacities.
Based on the above-described background, recording media that have higher sensitivities (that is, that can be recorded to at lower laser powers) are necessary for optical information recording media.
Several oxide materials have been conventionally proposed as write-once recording materials. For example, a recording material that uses TeOx, a Te low oxide, as its primary component is known. Among these, a Te—O—Pd recording material that uses a material in which Pd (Au and Pt can also be used) is dispersed among the TeOx as its primary component achieves a large signal amplitude and is thus known for its extremely high reliability (see Patent Citation 1). However, Te—O—Pd recording materials are problematic in that they cannot achieve a recording sensitivity of the degree required to meet the needs of the higher speeds and larger capacities of the future.
The following can be given as examples in which other materials are added to materials that use TeOx as their primary component in order to improve recording characteristics.
Patent Citation 2 proposes adding at least one compound selected from La—F, Mg—F, Ca—F, a rare-earth element fluoride, Si—O, Cr—O, Ge—O, Hf—O, Mo—O, Ti—O, W—O, Zn—O, and Zr—O to TeOx in order to obtain a higher signal quality by preventing thermal diffusion within the recording film surface in the case where a violet wavelength laser and an objective lens with an NA of 0.80 or more are used. While an improvement in the signal quality is obtained in such a case, there is no discussion of an improvement in the recording sensitivity.
Furthermore, although Patent Citation 3 discusses improving recording sensitivity by adding TlOx, a low-melting point material, this conventional technique provides insufficient effects with regards to increasing the sensitivity when recording information onto an optical information recording medium. In addition, although this patent citation also proposes increasing the change in the refractive index by adding BiOx, InOx, or the like, thereby increasing the optical change, the effect thereof on the recording sensitivity is not discussed.
Patent Citation 1: International Publication WO 98/09823
Patent Citation 2: JP 2005-22409A
Patent Citation 3: JP S55-38616A